XRF detectors are usually energy dispersive, with each incident X-ray producing a pulse whose charge is proportional to the energy of the X-ray. The detector electronics is designed to convert the pulses from multiple X-rays into a spectrum, which is a plot of X-ray energies vs the number of X-rays received with that energy. Such a spectrum will exhibit peaks at energies which correspond to the characteristic X-ray energies of elements within the sample being measured. The position, magnitude and width of the peaks are critical parameters enabling identification of the elements in the sample and determination of their concentration.
In order to ensure that test results are accurate and repeatable, it is important to avoid electronic drift of signals from the detector. Signal drift will result in X-rays of the same energy being assigned a different energy in the spectrum at different measurement times. The signal drift may cause misidentification of elements and/or errors in measurement of their concentration. There are two detector parameters whose stability is particularly important for avoiding signal drift: these are the detector temperature and the detector bias voltage.
XRF detectors are usually cooled below room temperature by means of a cooling unit which is thermally coupled with the detector body. Maintaining accurate control of the cooling unit so that the detector temperature does not drift is essential to avoid signal drift.
XRF detectors require application of one or more bias voltages. In the present disclosure, reference may be made to components for control of a single bias voltage, but it is to be understood that whenever bias voltage is referred to in the singular, similar components may be used to maintain control of multiple bias voltages, and such components and the use of multiple bias voltages are all within the scope of the present disclosure.
Maintaining accurate control of the bias voltage regulator, ensuring that the bias voltage does not drift, is essential to avoid signal drift.
Control of a cooling unit generally requires a temperature setpoint (usually a digital value), a digital-to-analog converter (DAC) to convert the setpoint to an analog signal, an analog temperature measurement signal, and a comparator producing an error signal of the temperature measurement relative to the setpoint. The error signal is then used as the control signal for the cooling unit.
Similarly, control of a bias voltage generally requires a bias voltage setpoint (usually a digital value), a DAC to convert the setpoint to an analog signal, an analog bias voltage measurement signal, and a comparator producing an error signal of the bias voltage measurement relative to the setpoint. The error signal is then used as the control signal for the regulator of the bias voltage supply.
The temperature measurement signal and the DACs each require an accurate and stable reference voltage. In existing practice, the reference voltage for the temperature measurement may be on the detector preamplifier printed circuit board (PCB), the reference voltage for the temperature setpoint DAC may be on the cooling unit control PCB, and the reference voltage for the bias voltage setpoint DAC may be on the bias voltage supply control PCB. This means that there may be three different voltage references, each with its own specific accuracy and drift, and the combined effect of uncertainty of the voltage references may cause significant degradation of the detector signal accuracy. Degradation of measured detector signal amplitude accuracy includes time dependent drift of the signal amplitude during a single measurement, drift of the signal amplitude between different measurements on the same instrument, and inconsistent measurements of the same or similar sample made on different instruments.
A further optional element for control of the temperature is use of an analog-to-digital converter (ADC) for converting the analog temperature measurement signal to a digital value for comparison with the temperature setpoint. Similarly, an ADC may be used to convert the analog bias voltage measurement signal to a digital value for comparison with the bias voltage setpoint. However, each of these ADCs also requires a voltage reference, and in existing practice the voltage references for the ADCs may be different from the voltage references for the temperature measurement and the DACs. This causes errors in the temperature and bias voltage setpoints, which result in still further inaccuracy and drift in the detector signal.